Design and Evaluation of a new MAC Protocol for Long- Distance - - PowerPoint PPT Presentation

Design and Evaluation of a new MAC Protocol for Long- Distance 802.11 Mesh Networks

by Bhaskaran Raman & Kameswari Chebrolu ACM Mobicom 2005

Reviewed by Anupama Guha Thakurta CS525M - Mobile and Ubiquitous Computing Seminar, Spring 2006

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OUTLINE

• Introduction
• Background
• Protocol Design and Implementation
• Topology Construction
• Evaluation
• Discussion and Conclusions
• Comments

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INTRODUCTION

• Motivations for new protocol:

– low cost internet access to rural areas – achieve performance improvement over 802.11 CSMA/CA in long distance mesh networks

• 802.11 CSMA/CA MAC was designed to

resolve contentions in indoor environments

• Use of wire-line, cellular or 802.16

currently prohibitive because of costs

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INTRODUCTION (Cont.):

Issues Addressed

• Find an alternative to 802.11 CSMA/CA MAC

protocol that allows simultaneous synchronous transmission / reception of multiple links at single node

• Propose a new MAC protocol: 2P

Cost advantages with off-the-shelf 802.11 hardware Show dependence of 2P on network topology Show that more UDP throughput than CSMA/CA is achievable (achieved 3-4 times) Show that more TCP throughput than CSMA/CA is achievable (achieved 20 times)

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INTRODUCTION (Cont.):

Mesh NW Characteristics

• Multiple radios

per node (one radio per link)

• High-gain

directional antennae

• Long distance

point-to-point links of several kilometers

Landline node

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OUTLINE

• Introduction
• Background
• Protocol Design and Implementation
• Topology Construction
• Evaluation
• Discussion and Conclusions
• Comments

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BACKGROUND

SynOp: Simultaneous Synchronous Operation (SynRx / SynTx)

• Syn-Rx: R1 and R2 receive simultaneously; Feasible
• Syn-Tx: T1 and T2 transmit simultaneously; Feasible
• Mix-Rx-Tx: R1 receives and T2 transmits; Not feasible

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BACKGROUND (Cont.):

SynOp: Simultaneous Synchronous Operation (SynRx / SynTx)

• In 802.11 Mix-Rx-Tx is not

feasible because of:

physical proximity and side lobes of directional antennae

• In 802.11 SynOp is feasible

but not allowed because:

SynRx: IFS based

immediate ACK mechanism SynTx: Carrier sense mechanism of interfaces give rise to backoffs

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OUTLINE

• Introduction
• Background
• Protocol Design and Implementation
• Topology Construction
• Evaluation
• Discussion and Conclusions
• Comments

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2P PROTOCOL DESIGN & IMPLEMENTATION

• SynOp is possible by

disabling ACK and Carrier sense mechanisms

• Simple Concept: each

node switches between SynRx & SynTx

• When a node is in

SynRx its neighbors are in SynTx phase and vice the versa

• Bipartite Topology

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2P PROTOCOL DESIGN & IMPLEMENTATION (Cont.):

• Solutions for SynRx in existing hardware:

Disable immediate ACKs’ by:

Independent Basic Service Set mode for interface

• perations, with separate SSID

Convert IP unicast pkts. to MAC broadcast pkts. at the driver level Send ACKs’ in the LLC implemented by the driver, by piggybacking them on data packets

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2P PROTOCOL DESIGN & IMPLEMENTATION (Cont.):

• Solutions for SynTx in existing hardware:

Disable carrier-sense backoffs by:

utilizing the two antennae connector feature provided by Intersil Prism chipset

How it works:

Select receiving antenna at driver level by antsel_rx command Connect external antenna to, say LEFT connector of radio card During transmission, the receiving antenna connector which is not connected to any external antenna is set to RIGHT This forces carrier-sense to happen on the RIGHT connector which sees only negligible noise Switch the receiving antenna to LEFT connector before switching from SynTx to SynRx

OVERHEAD?

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2P PROTOCOL DESIGN & IMPLEMENTATION (Cont.): Loose Synchrony

An interface sends B bytes in SynTx, then sends a marker packet as a “token” Enter the SynRx phase Switch to SynTx upon receiving a marker packet or upon timeout OVERHEAD?

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2P PROTOCOL DESIGN & IMPLEMENTATION (Cont.): Problems in Loose Synchrony

Temporary loss of synchrony (marker loss) Link intialisation (link recovery after failure)

Solution: On entering SynRx, ifa starts a timer to control timeout

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2P PROTOCOL DESIGN & IMPLEMENTATION (Cont.): Problems in Loose Synchrony

• Two ends of a link get out of synchrony

and timeout at the same time Solution: Add random perturbation (bumping) to the timeout value each time

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2P PROTOCOL DESIGN & IMPLEMENTATION (Cont.): Communication Across Interfaces

• Coordination of interfaces to switch from SynRx

to SynTx Once an ifa decides to switch to Tx, it sends a notification (NOTIF) to other ifa-nbrs’, and waits for NOTIF from them. Aware of UP / DOWN status of other ifa-nbrs’. (observation of 3 consecutive time-outs implies DOWN)

• Coordination of interfaces to switch from SynTx

to SynRx Not necessary since all ifas’ begin Tx simultaneously and with the same duration of B bytes

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OUTLINE

• Introduction
• Background
• Protocol Design and Implementation
• Topology Construction
• Evaluation
• Discussion and Conclusions
• Comments

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TOPOLOGY CONSTRUCTION

• Constraints in Topology

Bipartite Constraint:

• If a node is in SynRx its neighbors should be in

SynTx and vice versa

• Implies no odd cycles are present

Power Constraint: For proper reception we require that

• the signal level is above min. reqd. power level

Pmin

• SINR has to be above the interference by

SIRreqd

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TOPOLOGY CONSTRUCTION (Cont.):

• For a given

topology

Power transmission Pi’s, (i = 1,2,…NA) are variables d(i, j), distance between the nodes corresponding to antennae a i and a j is known g(i, j), effective gain when a i is transmitting and a j is receiving, is known

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TOPOLOGY CONSTRUCTION (Cont.): Power Equations Transmission power

Considered as interference from all

• ther nodes
• Eq. 1 and 3 are power equations.

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TOPOLOGY CONSTRUCTION (Cont.): Parameters in the Power Equations

• P_min: -85 dB for 11Mbps reception
• SIR_reqd: 10 dB for the 10-6 BER level, set to

14-16 dB in topology construction

• The antenna radiation pattern that decides the

gain in different angles.

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TOPOLOGY CONSTRUCTION (Cont.): Topology Formation

• Construct a tree topology that

satisfies the two constraints

– Suppose all (or most) traffic passes through the land-line node and don’t do multi-path routing – A tree rooted at the land-line node satisfies the bipartite constraint – Fault tolerance can be solved by morphing

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TOPOLOGY CONSTRUCTION (Cont.): Topology Formation

• Form a spanning tree with following

heuristics

– (H1) Reduce length of links used

• Interference and power consumption

– (H2) Avoid “short” angles between links

• Side-lobe leakage
• ang_thr of 30 to 45 degrees

– (H3) Reduce hop-count

• Deep trees = bad latency

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TOPOLOGY CONSTRUCTION (Cont.): Algorithm

1. Set of Unconnected nodes is U, set of all possible connection links is S, create links at hi 2. Order the links in S in increasing order of distance 3. For each link do

• angle threshold check: ignore if angle < ang_thr, else add
• Feasibility check (power constraint equation)

4. If all nodes connected, stop. 5. If successful in adding link in step 3, continue with step 1 6. If not successful in adding link in step 3, and link formed in hi, go to next link, go to step 1. 7. If not successful in adding any link, and no link formed for hi, declare failure, and stop.

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OUTLINE

• Introduction
• Background
• Protocol Design and Implementation
• Topology Construction
• Evaluation
• Discussion and Conclusions
• Comments

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EVALUATION: of topology creation

Purpose

The effectiveness of the algorithm The effect of varying the parameter SIRreqd

Evaluation subjects

4 collections of villages from a local district map

Q1, Q2, Q3 and Q4 Q1 has 31 nodes Q2-Q4 have 32 nodes, respectively

Topologies randomly generated

50 nodes in an area of 44Km X 44Km

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EVALUATION: of topology creation

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EVALUATION: of topology creation

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EVALUATION: of topology creation

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EVALUATION: simulation studies

• Goals:

To measure the impact that step by step link establishment has on loosely synchronized network Saturation throughput performance compared to CSMA/CA protocol Performance of TCP over 2P operated networks

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EVALUATION: extensions to ns-2

– ns-2 extended for:

• Multiple interface support
• Directional antenna support
• MAC modifications
• LLC modifications

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EVALUATION: Simulation results

• Link Establishment:

Method: add links one after another to an already synchronized network Results:

• Took 12.9ms for first link establishment
• Reason: first transmission of both ends of link

coincide and had to use bumping to establish link

• Took 4.9ms for rest of the links to establish
• No noticeable difference in throughput of

already synchronized links while adding new links

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EVALUATION: Simulation results

• Saturation

throughput

– UDP traffic

• One packet every

2ms

• Packet size: 1400

bytes

– Results:

• Nodes operated in 2P

achieve around 3-4 times more bandwidth than operated in the CSMA/CA protocol

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EVALUATION: Simulation results

• TCP Performance

In loss free: Up to 20 times better performance than CSMA/CA

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EVALUATION: Implementation based results

• Prototype implementation on HostAP v0.2.4 on

Linux v2.4.20-8

• Confirmation of SynOp with Prism2 cards:

6.5Mbps throughput on each link at the same time.

• 2P performance on a single link:

3.05Mbps average throughput – lower than 4.4Mbps observed in simulations Overheads of marker pkts. And changing of antsel_rx in Prism2 cards give a combined throughput of 6.1Mbps which is less than 6.5Mbps

• bserved.

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EVALUATION: Implementation based results (Cont.)

• Sub-optimal performance of 2P on a pair of links:

Per interface throughput is lower than 3.05 Mbps because contention window set at 32 instead of 1 hence random backoff even in the absence of carrier sense Limitations in driver level approach to 2P implementation Stress of CPU scheduling involved in copying of rx/tx bytes to/from hardware as PCMCIA cards used didn’t have Direct Memory Access

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OUTLINE

• Introduction
• Background
• Protocol Design and Implementation
• Topology Construction
• Evaluation
• Discussion and Conclusions
• Comments

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Discussion and Conclusions

• Prior work involves Spatial reuse Time Division

Multiple Access (STDMA) scheduling

• The present work differs in:

Multiple radios per node Directional antennae Exact location of nodes

• Fault tolerance and Morphing

Trees are not very fault tolerant Morph the topology in the event of a failure

• Provision additional links, but turn them on only as

needed Morphing can be used to create new routes when network equipment is turned off

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OUTLINE

• Introduction
• Background
• Protocol Design and Implementation
• Topology Construction
• Evaluation
• Discussion and Conclusions
• Comments

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COMMENTS

Pros: 1.Performance enhancement 2.Low cost implementation

• 3. Fault tolerance solution
• 4. Feasible protocol

Cons:

• 1. Requires one

dedicated transceiver for each link

• 2. Reconfigure on node’s

joining / removal / relocation

• 3. Topology is centralized

with multiple landlines

• 4. Transmit empty pkts –

fairness & security

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QUESTIONS